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LAB MANUAL/EMIT
EXPERIMENT NO.3
PART A
TITLE: Digital Storage Oscilloscope (DSO)
AIM: Perform following using DSO
1)Perform Roll, Average, and Peak detection operations on signal capture transient.
2) Perform FFT analysis of sine and square wave signals.
3) Perform various math operations like addition, subtraction and multiplication of
two waves.
4)Check store and retrieval of signals. Use Print, save on disk/USB.
APPARATUS:
Sr. No.
Instrument
1
DSO
2
Function Generator
3
DC Power Supply
4
Relay
Model
SPECIFICATIONS:
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THEORY:
The main purpose of an oscilloscope is to give an accurate visual representation of electrical signals. An
oscilloscope is a measurement and testing instrument used to display a certain variable as a function of
another.
Figure shows the block diagram of DSO as consists of,
1. Data acquisition
2. Storage
3. Data display.
Fig. 1 Block Diagram of DSO
Data acquisition is earned out with the help of both analog to digital and digital to analog
converters, which is used for digitising , storing and displaying analog waveforms. Overall
operation is controlled by control circuit which is usually consists of microprocessor.
Data acquisition portion of the system consist of a Sample-and-Hold (S/H) circuit and an analog
to digital converter (ADC) which continuously samples and digitizes the input signal at a rate
determined by the sample clock and transmit the digitized data to memory for storage. The
control circuit determines whether the successive data points are stored in successive memory
location or not, which is done by continuously updating the memories. When the memory is full,
the next data point from the ADC is stored in the first memory location writing over the old data.
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The data acquisition and the storage process is continues till the control circuit receive a trigger
ignal from either the input waveform or an external trigger source. When the triggering occurs,
the system stops and enters into the display mode of operation in which all or some part of the
memory data is repetitively displayed on the cathode ray tube.
In display operation, two DACs are used which gives horizontal and vertical deflection
voltage for the CRT Data from the memory gives the vertical deflection of the electron beam,
while the time base counter gives the horizontal deflection in the form of staircase sweep signal.
Types of oscilloscopes :
o
Analog oscilloscopes :
The first oscilloscopes were analog oscilloscopes, which use cathode-ray tubes to display a waveform.
The downside of an analog oscilloscope is that it can not “freeze” the display and keep the waveform for
an extended period of time. Once the phosphorus substance delaminates, that par tof the signal is lost.
Also, you cannot perform measurements on the waveform automatically. Instead you have to make
measurements by hand using the grid on the display.
o
Digital storage oscilloscopes (DSOs) :
Digital storage oscilloscopes (often referred to as DSOs) were invented to remedy many of the negative
aspects of analog oscilloscopes. DSOs input a signal and then digitize it through the use of an analog-todigital converter. Figure 1 shows the block diagram of DSO. The attenuator scales the waveform. The
vertical amplifier provides additional scaling while passing the waveform to the analog-to-digital
converter (ADC). The ADC samples and digitizes the incoming signal. It then stores this data in memory.
The trigger looks for trigger events while the time base adjusts the time display for the oscilloscope. The
microprocessor system performs any additional post processing specified by the user before the signal is
finally displayed on the oscilloscope. Having the data in digital form enables the oscilloscope to perform
a variety of measurements on the waveform.
Signals can also be stored indefinitely in memory. The data can be printed or transferred to a computer
via a flash drive, LAN, or DVD-RW. In fact, software now allows you to control and monitor your
oscilloscope from a computer using a virtual front panel.
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Modes of Operation
 Roll Mode
Roll mode is used to move waveform slowly across the screen from right to left. It only
operates on time base settings of 500 ms/div and slower. Roll mode is to be used for low
frequency waveforms.

Store Mode
In this case input initiates trigger circuit memory, write cycle starts with trigger pulse
when memory is full, write cycle stops. Then using digital to analog converter, the stored signal
is converted to analog and displayed. When next trigger occurs the memory is refreshed.

Hold or save Mode
This is automatic refresh mode. When new sweep signal is generated by time base
generator, the old contents get over written by new one, by pressing hold or save button,
overwriting can be stopped and previously saved signal gets locked.
Acquisition Modes
When you acquire a signal, the oscilloscope converts it into a digital form and displays a
waveform. The acquisition mode defines how the signal is digitized and the time base setting
affects the time span and level of the signal in the acquisition. There are three acquisition modes:
Sample, Peak Detect, and Average.

Sample: In this acquisition mode, the oscilloscope samples the signal in evenly spaced
intervals to construct the waveform. This mode accurately represents signals most of the time.
However, this mode does not acquire rapid variations in the signal that may occur between
samples. This can result in aliasing and may cause narrow pulses to be missed. In these cases,
Peak Detect mode can be used to acquire data.

Peak Detect: In this acquisition mode, the oscilloscope finds the highest and lowest
values of the input signal over each sample interval and uses these values to display the
waveform. Noise will appear to be higher in this mode.

Average: In this acquisition mode, the oscilloscope acquires several waveforms,
averages them, and displays the resulting waveform. This mode reduces random noise.
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Important Oscilloscope Performance Properties
a) Bandwidth and channels:
It dictates the range of signals (in terms of frequency) that can be accurately displayed and tested.
Bandwidth is measured in Hertz.. A channel refers to an independent input to the oscilloscope.
The number of oscilloscope channels varies between two and twenty. Most commonly, they have
two or four channels.
b) Sample Rate:
The sample rate of an oscilloscope is the number of samples the oscilloscope can acquire per
second. It is recommended that oscilloscope should have a sample rate that is at a least 2.5 times
greater than its bandwidth. However, ideally the sample rate should be 3 times the bandwidth or
greater. Manufactures typically specify the maximum sample rate an oscilloscope can attain, and
often this maximum rate is possible only when one channel is being used. If more channels are
used simultaneously, the sample rate may decrease.
c) Memory depth:
A digital oscilloscope uses an analog-to-digital converter to digitize the input waveform. The
digitized data is stored in the oscilloscope’s high-speed memory. Memory depth refers to exactly
how many records and, therefore, what length of time can be stored. Memory depth plays an
important role in the sampling rate of an oscilloscope. In an ideal world, the sampling rate would
remain constant no matter what the settings were on an oscilloscope. However, this kind of an
oscilloscope would require a huge amount of memory at small time/division settings and would
have a price that would severely limit the number of customers that could afford it.
Memory depth = (sample rate). (Time across display)
DSO features
a) Pre- trigger function: The DSO is capable of recording the waveforms preceding the triggering
point. It continuously stores data until a trigger occurs storing is stopped at the predefined no. of
sampling after the trigger &then the stored data is displayed with the trigger point as reference.
b) Observation of single shot events: A DSO can capture single _shot events such as power supply
start up characteristics, power resets, power failure detection, counter measures against noise &
instantaneous waveforms for areas that include mechanical equipments such as motors.
c) Large memory capacity: DSO stores the observed data in memory. Memory capacity is unlimited.
With a large memory capacity, phenomenon can be recorded over a long period.
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d) Computations: Since the collected waveform data is expressed as digital values, sophisticated
computation processing can be performed on the waveform data &the results are displayed on the
screen in real time. This enables various functions such as auto set up function.
e) Data output: Digitization of waveforms data allows various forms of output. For eg. By
incorporating a printer in the digital oscilloscope, the display on the screen can be immediately
printed out and time consuming.

Applications :
DSO finds various applications in a wide variety of fields:
a)
b)
c)
d)
e)
f)
It is used for testing signal voltage in circuit debugging.
Testing in manufacturing.
Designing purpose.
Testing of signals voltage in radio broadcasting equipment.
In the field of research.
Audio and video recording equipment.
CIRCUIT DIAGRAM:-
vi : input square wave from function generator.
v : output across C obeserved on Digital storage oscilloscope.
Figure 2: RLC circuit
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Figure 3.Transient response of second order system
1. Rise time (tr) :The time required for the waveform to go from 0.1 of the final value to 0.9 of the final
value.
2. Peak time(tp): The time required to reach the first, or maximum, peak.
3. Percent overshoot(Mp): The amount that the waveform overshoots the steady state, or final, value at
the peak time, expressed as a percentage of the steady-state value.
4. Settling time(ts): The time required for the transient's damped oscillations to reach and stay within
±2% of the steady-state value.
5. Delay time (td): The delay time is the time required for the response to reach half the final value the
very first time.
PROCEDURE:
1. Apply square wave input from function generator to RLC circuit.
2. Observe the output across capacitor using DSO.
3. Measure different delays and peak overshoot using cursor measurement.
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OBSERVATION TABLE:
SR. NO.
Rise time (tr)
Peak time(tp)
Settling time(ts)
Delaytime (td)
Percent
overshoot(Mp)
CONCLUSION:_______________________________________________________________________
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PART B
FFT Analysis
DSO can be used in FFT mode to find out the frequency components of the input signal.
In FFT Math mode, a time-domain signal is converted into its frequency components (spectrum).
The Math FFTmode can be used for the following types of analysis:

Analyse harmonics in power lines

Measure harmonic content and distortion in systems

Characterize noise in DC power supplies

Test impulse response of filters and systems

Analyse vibration
FFT is a convenient and powerful tool for performing frequency domain analysis on variety of
signals. FFT operates on discrete samples of signals. Thus it is important to understand the
relationship between sampling rate, Fourier Transform and Discrete Fourier Transform.
Sampling theory provides mathematical foundations for analyzing continuous time signals with
Digital Signal Processing (DSP) methods.
A Fourier Transform (FT)converts time (or space) domain into frequency domain;
ADiscrete Fourier Transform (DFT) converts a finite list of equally spaced samples of a function into
the frequency domain. In DFT there are so many operation performs, So that it required more time. Fast
Fourier Transform is used to perform DFT rapidly.
A fast Fourier transform(FFT) is an algorithm to compute the discrete Fourier transform (DFT) and its
inverse rapidly.
Since the DFT requires a finite length input signal, an “on-going” signal must be truncated
before FFT computations take place .The truncation process is achieved by overlapping input
sequence x[n] with a finite length window w[n] .FFT is computed on signal y(n),
Y (n) =x [n]*w [n]
The windowing process introduces a loss in spectral resolution and an effect known as Spectral
Leakage. In general, choice of window function with better ‘frequency resolution ‘, doesn’t, in
general perform as well in terms of ‘ spectral leakage’
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In one part of experiment we will employ following windows:
1) Rectangular window
2) Hanning window
3) Flattop window
4)
Displaying the FFT Spectrum
Push the Math button to display the Math Menu. Use the options to select theSource channel, Window
algorithm, and FFT Zoom factor. You can display onlyone FFT spectrum at a time.
Math FFT option
Source
Window
FFT Zoom
Settings
CH1, CH2
Comment
Hanning, Flattop, Rectangular
Selects the channel used as the FFT source
Selects the FFT window type;
X1, X2, X5, X10
Changes the horizontal magnification of the FFT display;
The Math FFT function includes three FFT Window options. There is a trade-offbetween frequency
resolution and amplitude accuracy with each type of window.
Window
Measure
Characteristics
Hanning
Flattop
Rectangular
Periodic waveforms
Periodic waveforms
Pulses or transients
Better frequency, poorer magnitude accuracy than Flattop
Better magnitude, poorer frequency accuracy than Hanning
Special-purpose window for waveforms that do not have
discontinuities.
FFT Aliasing
Problems occur when the oscilloscope acquires a time-domain waveform containing frequency
components that are greater than the Nyquist frequency.The frequency components that are above
theNyquist frequency are undersampled, appearing as lower frequency componentsthat "fold back"
around the Nyquist frequency. These incorrect components are called aliases.
Nyquist Frequency
In order to recover all Fourier components of a periodic waveform, it is necessary to use a
sampling rate at least twice the highest waveform frequency. The Nyquist frequency, also
called the Nyquist limit, is the highest frequency that can be coded at a given sampling rate in
order to be able to fully reconstruct the signal, i.e.
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PROCEDURE:
i) Effect of different window function
1. Connect a 3.5V (p-p) sine i/p signal at 1 KHz to DSO channel1.
2. Use ‘Autoscale’ to display time domain waveform.
3. Press MATH button.
4. Select FFT.
5. Adjust FFT menu settings.
6. Use Hanning, rectangular and flat top windows and observe the frequency response.
ii) Aliasing
The procedure given below is to demonstrate that aliasing occurs if the effective sampling rate is
below Nyquist rate.
1. Connect a signal generator to channel1. Select a sinusoidal / square wave signal i/p with V (pp) =3.5V and frequency = 10 KHz.
2. Select with FFT menu sampling frequency 5OKsa/s.
3. Use the cursors and find peaks function to measure peak frequency.
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4. Repeat the above procedure for different i/p frequency setting for signal i/p
(i)
25 Khz
(ii)
40 Khz
(iii)
70 KHz
OBSERVATION TABLE:
Fin = ______________
SR. NO.
Fsample
st
1 Harmonic
FFundamental
2ndHarmonic
1
2
CONCLUSION:
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